The present invention relates to signal processing in a frequency-modulated continuous-wave (FM-CW) radar processing system and, more particularly, to a pairing process whereby peak signals occurring in relation to a target object, and obtained by frequency-analyzing a beat signal occurring between a received signal and a transmitted signal, are paired between an up portion and a down portion of a triangular-shaped FM-CW wave.
An FM-CW radar system is mounted on a vehicle or the like, and measures the distance or relative velocity of a target object, such as a vehicle traveling in front, by transmitting a triangular-shaped frequency-modulated continuous wave. More specifically, the transmitted wave from the radar is reflected by a vehicle traveling in front, and a beat signal (radar signal) occurring between the received signal of the reflected wave and the transmitted signal is obtained. This beat signal is fast-Fourier transformed (FFT) to analyze the frequency. The frequency-analyzed beat signal exhibits a peak at which the intensity becomes large corresponding to the target object. The peak frequency corresponding to this peak carries information concerning the distance, and the peak frequency differs between the up portion and down portion of the triangular FM-CW wave due to the Doppler effect associated with the relative velocity of the vehicle in front. The distance and relative velocity of the vehicle in front are determined from the peak frequencies in the up and down portions. If there is more than one vehicle traveling in front, one pair of peak frequencies, one in the up portion and the other in the down portion, occurs for each vehicle. Forming the pair of peak frequencies between the up portion and the down portion is called the pairing.
In an FM-CW radar processing system, a triangular wave is often used as the modulating signal; that is, a frequency-modulated wave, modulated with a triangular wave, is transmitted, and a reflected wave from a target object is received and the received signal is frequency-demodulated using the frequency-modulated wave as the local frequency. Here, the received wave from the target is shifted in frequency from the transmitted wave (i.e., produces a beat) according to the distance between the radar antenna and the target and also to the Doppler shift due to the relative velocity of the target. Therefore, the distance and the relative velocity of the target object can be measured from this frequency shift.
As described above, signal processing such as a fast Fourier transform is applied to the transmission/reception beat frequency by a CPU, and processing is performed to pair up the peak frequencies, based on which the distance and the relative velocity are determined.
In the prior art FM-CW radar processing system described above, the pairing is done by simply combining those close in peak frequency or close in peak intensity in the up portion and in the down portion of the triangular wave or, in the case of a scanning system or the like in which angle information is available, by combining those close in angle in the up portion and in the down portion.
However, if difficult conditions occur, for example, if a plurality of similarly reflecting target objects are present, simply pairing up the peak frequencies appearing in the up portion and the down portion may result in an erroneous combination.
For example, when peak A1 should be paired with peak D1 and peak B1 with peak C1, the peaks may not be paired up as expected. The prior art system, however, cannot detect the occurrence of such mispairing. If the distance and relative velocity are calculated without correcting the mispairing, the intended results cannot be obtained; specifically, an erroneous calculation of the relative velocity would pose a problem as it seriously affects control operations such as vehicle speed control.
In one method practiced to address this problem, the derivative of the distance is compared with the relative velocity and, if they match, it is determined that correct pairing has been done; otherwise, it is determined that mispairing has been done. With this method, however, if the relative speed of the target object is small, it takes time to judge the correctness of the pairing, and this also seriously affects control operations such as vehicle speed control.
It is accordingly an object of the present invention to solve the above problem and provide an FM-CW radar processing system which can transmit a radar wave by switching between two different modulation frequencies, triangular wave frequencies, or center frequencies and detect the occurrence of mispairing by comparing the results of the pairing.
To achieve the above object, the present invention proposes a pairing process for an FM-CW radar processing system as described below.
That is, according to the present invention, there is provided an FM-CW radar processing system which transmits an FM-CW wave and receives a reflected wave from a target object, comprising: modulating signal control means capable of changing a modulating signal to be applied to the FM-CW wave; pairing means for pairing peak signals obtained by fast-Fourier transforming a beat signal occurring on an up portion and a down portion of the FM-CW wave; calculating means for calculating a distance or relative velocity with respect to the target object, based on the peak signal in the up portion and the peak signal in the down portion obtained by the pairing; and comparing means for comparing the distance or relative velocity obtained by the calculating means after changing the modulating signal with the distance or relative velocity obtained before changing the modulating signal.
In the above processing system, when the comparing means has detected that the distance or relative velocity differs before and after changing the modulating signal, the pairing is judged to be mispairing.
The modulating signal control means changes a modulation frequency or frequency period for the modulating signal, and the calculating means calculates the distance to the target object after the change of the modulating signal; further, the modulating signal control means changes a center frequency for the modulating signal, and the calculating means calculates the relative velocity with respect to the target object after the change of the modulating signal.